Ethylene copolymers, especially copolymers of ethylene with propylene optionally in combination with non-conjugated diolefins, are widely used as thermoplastic polymers, as elastomeric polymers in both vulcanized and unvulcanized compositions, in blends and as viscosity modifiers in lubricating oil formulations. One example of the use of these materials as viscosity modifiers is disclosed in U.S. Pat. No. 6,589,920.
The elastomeric properties of ethylene copolymers depend largely on the ethylene content of the copolymer since materials with higher ethylene contents have a lower entanglement molecular weight. The solution properties also depend to a large extent on the ethylene content; with a low ethylene content EP copolymer being desirable when the copolymer is intended for use as a viscosity modifier for waxy base stock oils because this provides excellent low temperature properties while maintaining good thickening efficiency. In addition, for many applications, low molecular weight EP copolymers are needed, for example to meet shear stability standards when used in motor oil formulations.
However, EP copolymers with low ethylene content, e.g., 40 to 55 weight percent (wt. %) ethylene, and low molecular weight, e.g., below 100,000 g/mol, are currently very difficult to produce and handle, particularly using most metallocene catalysts systems. Thus, these materials tend to be amorphous and have a propensity to agglomerate or cold flow and stick to finishing equipment.
Ethylene copolymers having a high ethylene content, e.g., ranging from 70 wt. % to 90 wt. %, are also desirable because of their outstanding low temperature impact strength due to low crystallinity, low modulus, and high flexibility. Furthermore, metallocene based EP copolymers with ethylene contents ranging from 70 wt. % to 90 wt. % show very good organoleptics, i.e., low odor/taste/extractables, due to their narrow molecular weight distribution as compared to Ziegler/Natta based copolymers. However, these copolymers are also difficult to handle and may have an unacceptably low service temperatures due to the inherently low melting point of the random copolymers produced using most metallocene-based and Ziegler/Natta-based catalyst systems.
There is therefore, a need for ethylene copolymers having both a low ethylene content and a low molecular weight that can be handled in conventional processing equipment without the problem of agglomeration during finishing, packaging and transportation. In addition, there is a need for ethylene copolymers that have a high ethylene content in combination with a low molecular weight and that exhibit an improved melting point without loss of their low temperature properties.